Plasma spraying of alumina: Plasma and particle flow fields

A comprehensive experimental examination of the interaction between a subsonic thermal plasma jet and injected alumine, particles is presented. Measurements of plasma velocity, temperature, and entrained air were obtained from an enthalpy probe and mass spectrometer combination. A diffusive separation, or demixing, of the Ar and He plasma gas was observed. Centerline plasma velocities and temperatures ranged from 1501500 m/s and 2000 to 14,000 K, respectively, in the region between the torch and a typical substrate location of 90 mm. Measurements of particle size, velocity, tempearture and local number density were obtained from a combination laser particle sizing system, Laser doppler velocimeter (LDV), and two-color pyrometer. Centerline temperatures and velocities for the nominally 30 m particles used were 2400–2800 K and 150–200 m/s, respectively.     

In-flight particle diagnostics in induction plasma processing

A laser Doppler anemometer combined with a particle-emission spectrometer, are used for the study of the induction plasma spraying process. For this, the effects of chamber pressure, spray distance and torch nozzle design on the particle surface temperature and velocity as well as the fraction of hot particles included in the stream of processed material, were investigated. A comparison between the velocity measurements by laser Doppler anemometry (LDA) and by the particle time-of-flight technique is presented in order to emphasize the deference between the velocity of the hot particles, and that of the total particle population, cold and hot. The influence of the individual particle mass on particle entrainment in the plasma jet from the ambient atmosphere in the vacuum chamber is discussed.     

Grain characterization of nanostructured TiN coatings prepared by reactive plasma spraying

Nanostructured TiN coatings were prepared by reactive plasma spraying, and the microstructure and the grain morphology of the coatings were analyzed by XRD, SEM and TEM. The results show that the coatings were mainly composed of TiN and Ti₃O phases (other phases were not detected), and they had the typical sprayed lamellar structures with few pores being visible. In the planar view, nanoscale grains with particle diameters ranging from 60 to 120 nm were observed in the coatings, and both fine equiaxial and columnar grains were found in some zones of the coatings. But in the cross‐sectional view, the TiN coatings were mainly composed of columnar grains, with no qualities of equiaxial grains being found. Thus, it can be concluded that the TiN coatings are composed of columnar grains, and the columnar grains are nanostructural, equiaxial grains in their cross‐section. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of aqueous aging on surface properties of plasma sprayed oxide coatings

The influence of aging in mild aqueous conditions (pH 4, 7 and 9) on surface properties of plasma sprayed oxide was studied using electrophoretic mobility studies and measuring concentrations of dissolved species from exposure liquids. In addition, required acid/base additions to maintain constant pH, redox potentials suspension conductivities were measured. The experiment time was two weeks. The plasma sprayed materials were based on Al(2)O(3), TiO(2) and Cr(2)O(3). Materials based on Al(2)O(3) dissolved easily at pH 4 due to presence of metastable gamma-Al(2)O(3) phase. In addition there was clear change in surface charging properties (zeta potential) of Al(2)O(3) surfaces so that the estimated IEP value drifted from >9 at the beginning of aging and dropped down to 8.5-8.7 after 2 weeks of treatment. Plasma sprayed TiO(2) did not dissolve under the experiment conditions. Even thought the surface charging (zeta potential) changed during the exposure, the estimated IEP remained close to the values reported for pure TiO(2) materials. Plasma sprayed Cr(2)O(3) based materials were also insoluble at the studied pH values. On the other hand, the estimated IEP values deviated radically from the reported PZC values of similar materials.     

Transport properties and microstructure changes of alumina coatings characterized by emanation thermal analysis

Emanation thermal analysis was used to characterize the thermal behaviour of alumina coatings as deposited on EUROFER 97 steel surface by filtered vacuum arc technique. Temperature ranges of the healing of cracks and structure irregularities observed by SEM were determined from the ETA results. Transport properties of the alumina coatings were assessed from the ETA results by the evaluation of radon diffusion parameters in the temperature range from 50 to 300°C. Healing microstructure irregularities of the alumina coatings can be expected in the range 300–700°C as indicated by the decrease of the radon release rate. From the ETA results it followed that the onset of healing the cracks observed by the SEM on the surface of one alumina coating sample can be expected at 430°C. Dedicated to Dr. K. Habersberger, Past-Chairman of the thermal analysis working group of the Czech Chemcial Society, at the occasion of his 75^th birthday     

Electron temperature and radiative attachment continua in enclosed inductively coupled plasma in argon and chlorine

Inductively coupled plasma (ICP) discharges in chlorine, argon, and their mixtures sustained inside a spherical quartz container at atmospheric pressure have been investigated. Continua of radiative attachment of a free electron to a chlorine atom in ICP are elucidated and are utilized to derive a spatial profile of electron temperature. A qualitative picture of elementary processes in enclosed ICP is given. Differences between electron and gas temperatures are discussed. Electron temperature is maximum in a periphery layer near the induction coil that is chosen for impurity determination. Concentrations of carbon and metal impurities are estimated.     

Microstructure and surface residual stress of plasma sprayed nanostructured and conventional ZrO2‐8wt%Y2O3 thermal barrier coatings

The nanostructured agglomerate feedstock which can be used for plasma spraying was obtained successfully by the reconstituting nanoparticle technique. Nanostructured and conventional ZrO₂‐8wt%Y₂O₃ (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on NiCoCrAlY coated 45# steel substrates. The microstructure and phase composition of feedstock and corresponding coating were characterized by SEM and XRD. The elastic modulus was tested by the nanoindenter instrument. The surface residual stresses were examined by XRD stress test equipment. The numerical value probability statistics of surface residual stress by the Weibull distribution theory was implemented under different thermal spray parameters. The value of residual stresses increased with increasing the thickness of the ceramic coating. The surface residual stress of nanostructured 8YSZ coating is lower than that of the conventional 8YSZ coating. A physical mechanical model is put forward to explain the causation of the variation of residual stresses. At the same time, the variation characteristic of surface residual stress is discussed by introducing the closed form solution of residual stress of laminar layer plate. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of calcination temperature on surface acidity of the solid superacid of sulfated alumina

The acidity of sulfated alumina catalysts was studied by analyzing DTG and heat of Ar adsorption together with the isomerization of pentane. The initial acid sites were Brönsted-type, and converted into Lewis-type upon increasing the pretreatment temperature. The heat of Ar adsorption of the most active sulfated alumina was 18.9 kJ mol^–1, this value being a little smaller than that of sulfated zirconia (23.6 kJ mol^–1).This revised version was published online in December 2005 with corrections to the Cover Date.     

Homogeneity studies of powders and plasma sprayed deposits

For production of plasma sprayed protective deposits and self-standing ceramic parts many different feedstock powders are used. The powders are very often of a composite type prepared by various technologies and mostly chemically very inhomogeneous. A method has been developed for evaluation of chemical homogeneity of both, the feedstock powders and the sprayed materials. The backscattered electron images along with the element mapping of dopant have been used as the basic measurement. The homogeneity coefficients were determined by quantitative point analysis with ZAF corrections. The results show that under all spraying parameters there is always a certain redistribution of all species leading to a better homogeneity. But generally, the more homogeneous the feedstock powder, the better the homogeneity of the deposits on both micro and macro scale.     

Simple and quick rotational temperature determination in N2-containing discharge plasma

A quick spectroscopic method of rotational temperature determination based on the low-resolution emission spectrum of the N₂ (v=2v=()) vibronic band profile of the second positive system iras developed. The new method of molecular gas temperature determination was used in an rf glow discharge excited in N₂ and N₂ + Ar + B₂ H₆ gas mixture at u pressure range 500–13011 Pa. This method of molecular gas temperature measurement is applicable to nonisothermic plasmas used in plasma-chemical reactors.     

Analysis of glass and glass melts during the vitrification process of fly and bottom ashes by laser-induced plasma spectroscopy. Part I: Normalization and plasma diagnostics

For laser-induced plasma spectroscopy (LIPS) analysis of the main components (Si, Al, and Ca) in glasses utilized for vitrification of ashes from waste incineration, a normalization procedure for line ratios is presented. Even in homogeneous glass samples, considerable pulse-to-pulse variations of the plasma electronic excitation temperature and electron density were observed because of changes in the material–laser interaction. A normalization procedure is outlined using Saha–Boltzmann equilibrium relationships to include the electronic excitation temperature and density in the calibration model. As a result of the normalization, the variation of the line ratios is reduced and linear calibrations for LIPS intensity ratios versus concentration ratios are achieved. For samples with high aluminum concentrations, the analysis was hampered by self-reversal effects.     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

Parametric study of an atmospheric pressure microwave-induced plasma of the mini MIP torch — II. Two-dimensional spatially resolved excitation temperature measurements

Two-dimensional mapping of the excitation temperatures have been carried out in the microwave-induced plasma torch with tangential argon flow. It is detected that the presence of wet aerosols in a nebulizer gas increases axial excitation temperature while desolvation acts in the opposite direction. The influence of low hydrogen concentration in the support gas and potassium in the nebulizer gas on the temperature distribution is studied in detail. The comparison of excitation temperature and electron number density distributions clearly indicates non-equilibrium plasma conditions.     

Influence of surface properties of particles on their adhesion and removal

The effect of surface properties of particles on their adhesion and removal was investigated using an immersed system consisting of nylon particles and a quartz plate. The nylon particles were dyed with a reactive dye in order to change their properties and were used for the adhesion and removal experiments in comparison with undyed particles. The electrokinetic potentials of the particles were measured by micro-electrophoresis and the Hamaker constants were independently evaluated using experimental values of dispersive component of surface free energy determined by the Wilhelmy technique. The experimental results were used for the discussion of particle adhesion and removal on the basis of the heterocoagulation theory. The differences in adhesion and removal efficiencies between dyed and undyed particles were explained in terms of the electrostatic and dispersive van der Waals interaction by considering the differences in thier properties, the electrokinetic potential and the Hamaker constant, due to dyeing.     

A Computational Examination of the Sources of Statistical Variance in Particle Parameters During Thermal Plasma Spraying

Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition(ZrO₂) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity, and direction) is introduced ``one phenomenon at a time to distinguish and characterize its effect and enable comparisons of separate effects. Calculations are also performed wherein all phenomena are considered simultaneously to enable further comparisons. Both nonswirling and swirling plasma flow fields are considered. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is mostsignificantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.     

H atom plasma diagnostics: A sensitive probe of temperature and purity

Two-photon absorption laser-induced fluorescence (TALIF) has proven to be a convenient diagnostic for reactive light atoms in plasmas. We have carried out a series of TALIF experiments and report the first temperature measurements of ground state H atoms in an rf discharge. With reasonable care, measurements of the H atom linewidths, broadened by the Doppler effect, provide detailed information about the translational energy, i.e., temperature of the atoms. It is found that in pure H₂ plasmas, the H atom temperature is slighthy elevated with respect to ambient. In plasmas contaminated with the other H-containing molecules, Doppler-broadened linewidths corresponding to H atom temperatures in excess of 7000 K have been observed. The mechanisms leading to such high apparent temperatures are discussed.     

Effect of annealing on hydrophobic stability of plasma deposited fluoropolymer coatings

Fluorinated amorphous carbon (a-C:F) films e.g. plasma polymerised perfluorocyclobutane have long attracted much consideration due to their low surface energy, hydrophobicity, low refractive index, good electrical and thermal insulation and good thermal stability. Although a-C:F films have many advantages, hydrophobic stability over time in air and water remains a major concern. In this study, the effects of weathering conditions on the hydrophobicity of fluorocarbon films prepared from perfluorocyclobutane precursors were examined using water contact angle measurements. It was found that the high initial hydrophobicity of as-deposited films degrades rapidly in humid conditions. The stability of hydrophobicity can be significantly improved when a suitable treatment such as annealing is employed. The mechanism of weathering was explained with the help of a number of morphological and chemical characterisation techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). In particular, XPS results demonstrated that a reduction in the overall amount of -CF₃ radical, oxygenation of surface fluorides and the formation of an overlayer all influence the degradation of fluorocarbon in aquatic environment.     

The effect of iron phosphate,alumina and silica coatings on the morphology of carbonyl iron particles

Carbonyl iron powders were coated with iron phosphate using phosphating method and boehmite (γ‐AlOOH) or silicon hydroxide (Si(OH)₄) nanoparticles derived from the hydrolysis of tri‐sec‐butoxide (Al(OC₄H₉)₃) or tetramethylsilane (Si(OCH₃)₄) using sol–gel method. The coated powders were dried and calcined at 400 °C for 3 h in air. Cross‐section morphology of coated carbonyl iron powders were investigated by scanning electron microscopy energy dispersive X‐ray analysis. Coated Fe micro‐particles were spherical in shape with ‘shell/core’ structures. The shells consisted of an amorphous layer with varying thickness (100–800 nm) and the core represented a carbonyl iron. Gelatinous morphology of dried FePO₄ coating composed from nanoparticles of iron oxyhydroxides and hydrated iron phosphate with a shell thickness of ～100 nm around iron particles was observed. In coatings based on alumina or silica xerogels with a thickness of ～100–150 nm or ～200–500 nm, the coatings were composed of iron oxyhydroxides and γ‐AlOOH or Si(OH)₄. The resulting XRD diffractograms revealed the hematite (α‐Fe₂O₃) and magnetite (Fe₃O₄) that were formed in phosphated and sol–gel coated iron powders. The X‐ray diffraction patterns did not verify the presence of phosphates, alumina or silica and indicate the amorphous or nanocrystalline structure of FePO₄, γ‐Al₂O₃ and SiO₂. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of the precursors on the properties of alumina supported rhodium particles

The effect of chlorine on the properties of Rh particles supported on alumina was studied using the hydroconversion of alkanes as a molecular probe. Chloride ions on the alumina have little influence on the Rh particles. Chlorine adsorbed on the Rh particles decreases the chemisorption capacity and increases the metal-support interaction, which results in modifications of their catalytic properties. Depending on the alkane processed, specific activity can be depressed up to four orders of magnitude.     

Weatherability and wear resistance characteristics of plasma fluoropolymer coatings deposited on an elastomer substrate

The weathering characteristics of thin film formed from plasma polymerisation of perfluoro(methylcyclohexane) on an EPDM substrate under cold plasma process operated at 13.56 MHz was investigated using an ATLAS Ci 3000 Xenon weatherometer. The effects of weathering conditions on the chemical composition of the substrate and the coating were examined using photoacoustic Fourier transform infrared spectroscopy (PA-FTIR) and X-ray photoelectron spectroscopy (XPS). The wear behaviour was examined using scanning electron microscopy (SEM). The change in the surface morphological behaviour of the coating indicates that the mending line of the patch-wise coating deposition or the fissure/crack line of the coating is particularly sensitive to the initiation of decomposition. FTIR and XPS spectroscopic investigations confirm that under humid and UV conditions, elimination of fluorine and introduction of new oxygen-containing functional groups play predominant role in the decomposition of the coating. Plausible mechanisms of degradation for the elastomer and the coating have been proposed. The coated substrate shows superior abrasion resistance characteristics with respect to the neat elastomer. The adhesion between the substrate and the plasma polymer coating appears to be excellent and remains strong after weathering.